Field of the Invention
The present invention relates to high energy laser systems, to beam delivery systems, and to laser peening systems suitable for use with stationary targets.
Description of Related Art
The use of mechanical shocks to form metals and to improve their surface properties has been realized for ages. In current industrial practice, a peening treatment of metal surfaces is most often accomplished by using high velocity shot. Treatment improves surface properties and, very importantly for many applications, results in a part displaying significantly improved resistance to fatigue and corrosion failure. A wide range of work pieces are shot peened in the aerospace and automotive industries. However, for many applications, shot peening does not provide sufficiently intense or deep treatment, does not provide sufficient control of intensity or depth, or cannot be used because of its detrimental effect on the surface finish.
With the invention of the laser, it was rapidly recognized that the intense shocks required for peening could be achieved by means of a laser-driven, tamped plasma. B. P. Fairand, et al., “Laser Shot Induced Microstructural and Mechanical Property Changes in 7075 Aluminum,” Journal of Applied Physics, Vol. 43, No. 9, p. 3893, September 1972. Typically, a plasma shock of 10 kB to 30 kB is generated at metal surfaces by means of high energy density (about 200 J/cm2), short pulse length (10-30 nanoseconds) lasers. A thin layer of metal tape, black paint or other absorbing material on the metal surface is sometimes used to provide an absorber to prevent ablation of the metal. A confining or tamping material such as water covers the surface layer providing an increased intensity shock. These shocks have been shown to impart compressive stresses, deeper and more intense, than standard shot peening. In testing, this treatment has been shown to be superior for strengthening work pieces from fatigue and corrosion failure. Laser peening is also used for forming and texturing surfaces.
One laser system which has been utilized for this purpose is described in our prior U.S. Pat. No. 5,239,408, entitled HIGH POWER, HIGH BEAM QUALITY REGENERATIVE AMPLIFIER. The laser system described in the just cited '408 patent comprises a high power amplifier in a master oscillator/power amplifier MOPA configuration capable of producing output pulses greater than 20 Joules per pulse with the pulse width on the order of 10 to 30 nanoseconds or less using a wavefront correcting configuration based on a stimulated Brillouin scattering (SBS) phase conjugator/mirror system.
In most commercial high energy laser peening processes, the laser beam position is held at a fixed location. The work piece being treated is moved relative to the laser beam line to create the applied spot pattern while maintaining the desired incidence angles, spot sizes, and orientations. This requires automation and work piece holding fixtures to grip the work piece and move it through its programmed positions. This method becomes both costly and highly work piece specific, requiring considerable engineering to develop processes for new work pieces. In addition, work piece size is limited to the lifting and handling capacity of the automation utilized. Work pieces and structures larger than automation handling capacity (for example, >1 m and/or >100 kg) cannot be laser peened by the conventional work piece moving approach.
Flexible beam delivery systems are often based on the use of optical fibers. However, even at wavelengths where glass fiber transmission is normally high, the very high pulse energy and high peak power used in laser peening can damage the fibers and render them ineffective. For example, a 25 J pulse is 100 times the maximum pulse energy (250 mJ) that can be delivered through a 1 mm multi-mode fiber. For single frequency beams, such as used in representative laser peening applications, glass fibers have even lower damage thresholds. U.S. Pat. No. 7,718,921, entitled ACTIVE BEAM DELIVERY SYSTEM WITH VARIABLE OPTICAL PATH SEGMENT THROUGH AIR, by Dane et al. (published 18 May 2006, as US 2006/0102602 A1) describes a flexible beam delivery system utilized for laser peening in industrial settings, where the target can be stationary while the laser pulses needed for laser peening are delivered with precision to the surface.
The system of the '921 Patent uses a transmitter mirror in a gimbal mount which directs the output beam across a free air path to a laser delivery tool that comprises an optical assembly (referred to herein as the “stinger”) which is held by an industrial robot. A receiver gimbal on the stinger keeps the laser beam aligned to the optical axis of the stinger, allowing the process robot to point and scan the stinger across the surface of the work piece, generating a well-defined pattern of laser peening spots. This has been a successful system in operation, used to process components such as engine fan blades for commercial aviation, steam turbine blades for power generation, and large blisks for aircraft. However, there are a number of disadvantages to its design, including for example:                1. The pointing accuracy of the stinger depends on the absolute accuracy of the robot axes; particularly the axes in the robot wrist. This often results in errors in the spot pattern placement that must be manually corrected in a process that requires multiple iterations and can take hours, days, or even weeks in some instances. Fortunately, once the pattern is established, the robot motion is very consistent, usually making it unnecessary to repeat this alignment exercise so long at the work piece and robot remain stationary.        2. The robot must reposition the stinger on every laser pulse to point to a new spot on the work piece, making the pulse repetition frequency (PRF) limited by the speed of the robot motion. The idea of applying more than one pulse for each stinger position is mentioned in the '921 Patent at column 11, line 48-column 12, line 12. However, the effectiveness of the approach mentioned there was limited by the small scanning range available, and would not be effective for complex surfaces.        3. There is no aspect ratio control to correct for spot elongation during off-axis peening. This means that non-normal beam incidence results in a rectangular (instead of square) spot shape which can have an aspect ratio as high as 3:1 near an incidence angle of 70 degrees (measured from the surface normal). For small spot areas this can result in a narrow beam foot print. Since peening efficiency is better for larger spot dimensions that create a flatter wave front for the shock wave induced in the metal, the narrow beam foot print reduces peening efficiency.        4. Active robot motion in the near proximity of a very valuable work piece can increase the possibility of a robot collision and damage to a customer part.        5. The stand-off distance between the final optic of the stinger and the treatment plane needs to be kept as short as possible to minimize the amount of robot motion required to hit different non-parallel surfaces on the work piece at near normal incidence.        6. The calibration of the energy and beam profile diagnostics built into the stinger is very sensitive to beam depolarization in the beam delivery path between the laser output and the stinger.        7. There is limited polarization control. In the '921 Patent, the beam polarization was fixed with respect to stinger orientation. A 90° quart rotator that can be moved in and out of the output beam by a pneumatic stage was used in later systems, but the polarization still could not be set to an arbitrary angle with respect to the work piece. This is particularly important for off-axis peening where reflections from the surface of the tamping water flow could result in significant loss of power that could be translated into the peening shock wave.        8. The diagnostic beam splitters on the stinger are susceptible to the generation of weak optical ghosts which can cause spurious signals on alignment cameras and the energy meter.        9. The stinger in industrial applications used a conventional mechanical design with a solid aluminum breadboard and optical components held with standard off-the-shelf mounts. This results in a heavy assembly which exceeded the recommended load capacity of the processing robot holding the stinger.        10. An unsealed optical enclosure on the stinger lead to frequent contamination of optical surfaces. This increased the need for routine inspection and cleaning of the optical components since dust and debris on optical surfaces can lead to catastrophic laser damage.        
In one adaptation of the system of the '921 Patent, has been used to laser peen form relatively flat panels. The flat panel system was a fixed processing cell that scanned the laser peening pulses over process areas of up to 48×48 in2 from a single receiver gimbal position on the stinger. This flat panel system used a scanning mirror installed on a motorized gimbal mount to move the spot across the treatment area. Using a single gimbal position encounters problems because of the range of angles of incidence in the process area as the beam scanned over the area 4 feet on a side. Thus, for the panel forming process, the challenge to overcome was how to maintain correctly located, rotated, shaped, and sized spots, independent of location on the panel.
Since the laser beam is converging on its way to the surface of the work piece, increasing propagation distance as the beam is pointed away from the center of the panel causes it to shrink. Non-normal angles of incidence cause the beam to elongate on the surface along the plane of incidence. Finally, if the plane of incidence on the scanning gimbal is not orthogonal to the square beam, spot rotation on the surface of the work piece will result.
To overcome these distortions, the flat panel system used a zoom telescope to adjust the beam divergence angle and a tilting telescope lens to pre-adjust the aspect ratio. A field-rotator was used to rotate the beam profile on target to compensate for out-of-plane gimbal mirror reflections. This allows a uniform pattern of square spots to be accurately placed on the work piece, as illustrated with respect to an example process field with reference to FIGS. 1(a) to 1(e).
FIG. 1(a) shows spot locations A through I which correspond to different locations in a field, where E is the normal incidence, center spot which is a square. A converging laser beam is directed across a 48″ square treatment field from a stationary gimbal location 66″ from the work surface. The corner spots A, C, I and G are 74.2″ from the gimbal. The spots B, F, H and D on the sides are 70.2″ from the work surface. With no corrections, the increased propagation distance and off-normal angles away from center would produce the spot shapes and sizes shown in FIG. (b). Rotating the spots using field rotator in advance of the gimbal, yields the symmetrical pattern shown in FIG. 1(c). By adjusting the output divergence using the zoom telescope, the spot areas can be made to have equal area as shown in FIG. 1(d). Finally, by tilting a zoom lens element, the relative horizontal and vertical divergence is controlled to generate the uniform pattern of spots as shown in FIG. 1(e). The flat panel system was suitable for delivering pulses across a large essentially flat process area, with relatively small range of angles of incidence on the panel for the spots in the pattern. However, it does not address the problems outlined above for implementation of a versatile system usable with complex surface geometries encountered in industrial laser peening systems.
It is desirable to provide a system that provides sufficient flexibility to be able to treat large work pieces and work pieces “in situ” at customer facilities, like aircraft or aircraft parts at an aviation repair station or large oil drilling work pieces at a pipe yard, and systems that overcome one or more of the problems outlined above.